Circuit assembly for operating a luminous signal

ABSTRACT

The invention relates to a circuit assembly for operating a luminous signal, particularly an LED signal, comprising at least one light-emitting diode (D) to which a resistance (R) is serially connected and a control (St) is connected in parallel. In order to adjust the luminous power of the luminous signal to various degrees of brightness during the day and at night, the control (St) is provided with a controlled source of current so as to specify a parallel current (I_P) which reduces the current (I_D) flowing through the light-emitting diode (D).

CLAIM FOR PRIORITY

This application claims priority to International Application No.PCT/DE02/04505 which was published in the German language on Jun. 26,2003, which claims the benefit of priority to German Application No. 10164 561.9, which was filed in German language on Dec. 14, 2001.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a circuit assembly for operating a luminoussignal.

BACKGROUND OF THE INVENTION

Illuminated signs based on light-emitting diodes instead of incandescentlamps are being used increasingly in many areas, especially forsignaling. Light-emitting diodes are comparatively cost-effective, havea long life and produce a strong light. U.S. Pat. No. 5,939,839discloses a circuit for protecting the light-emitting diodes of a lightsignal against overvoltages. European Application No 0 293 921 relatesto a circuit which protects light-emitting diodes against overheating.However, it is difficult to use light-emitting diodes where incandescentlamps are intended to be replaced by LED illuminated signs without anychanges to the drive. This is particularly true of light signal circuitsfor railroads, in which correct operation is generally monitored bycurrent measurement which is safe from the signaling point of view. Inorder to allow this monitoring still to be used without any changes, thecurrent/voltage characteristic of the LED illuminated sign mustcorrespond approximately to that of an incandescent lamp.

A further special feature is signaling outside tunnels or environmentalconditions with approximately constant light characteristics. In thiscase, the circuitry must provide a reduction in the light power fornight time operation, in comparison to that for daytime operation. Thelight sensitivity of the human eye during daytime differs by a factor ofapproximately 1000 from the light sensitivity at night. If the lightpower is not reduced at night, dazzling is therefore possible even inthe case of lights which are barely perceptible in the daytime. However,it is essential to avoid dazzling, particularly for road or railtraffic, since there is a risk of other signals effectively possiblybeing overlooked due to the excess radiation. In the case of lightsignals based on incandescent lamps for railroad purposes, thebrightness is controlled between daytime and night by the controlmechanism using the supply voltage or the supply current. Since thelight power of an incandescent lamp depends exponentially on the supplyvoltage or the supply current, a small change in the supply current orthe supply voltage leads to a major change in the light power. Thismeans that the supply current or the supply voltage need be reduced onlyto about ⅔ of the initial value in order to reduce the light power, forexample, to 20% of the initial light intensity. In order to achieve asimilarly advantageous characteristic profile for light-emitting diodes,a proposal has been made, according to DE 198 46 753 A1, for a drivecircuit to be connected in parallel with each light-emitting diode. Thishas the disadvantage that the achievable difference between the lightpower in the daytime and the night is relatively small. Furthermore,component tolerances which lead to different forward voltages on thelight-emitting diodes, transistors and other components, as well as theinfluence of temperature on the forward voltages, are not compensatedfor.

SUMMARY OF THE INVENTION

The invention discloses a circuit assembly which allows an increaseddynamic range between daytime and night light power.

According to one embodiment of the invention, by presetting the parallelcurrent which does not flow through the light-emitting diode, the lightpower of the light-emitting diodes can be controlled over a very widerange. In this case, in order to allow a constant light-emitting diodecurrent in spite of different operating voltages during daytime andnight time operation, a second controlled current source for presettingthe light-emitting diode current is provided.

However, it is also possible to achieve a constant current flow of thelight-emitting diode without any further controlled current source. Thisis done by controlling the parallel current as a function of theoperating voltage, rather than by keeping it constant.

According to another embodiment of the invention, a nominal/actual valuecomparison is carried out by means of a comparator for the parallelcurrent to be controlled and, if appropriate, also the light-emittingdiode current to be controlled. The nominal value of the respectivecurrent is in this case preset by predefined switching thresholds, whichpredetermine when the corresponding currents should be switched on andoff. The switching thresholds may be analogue continuous presets or elsepurely digital information, which then preferably has hysteresis.

In one advantageous embodiment of the invention, the switching thresholdhas temperature compensation and/or forward voltage compensation appliedto it. The temperature compensation compensates for componentcharacteristics which vary as a function of the temperature. The forwardvoltage compensation compensates for different forward voltages of thelight-emitting diodes that are used. For example, it is feasible for thelight-emitting diodes which are used to be subdivided into forwardvoltage groups, from which the appropriate light-emitting diodes to befitted are selected.

In another embodiment of the invention, the assembly can produce acurrent even at low voltages. This function protects the circuitarrangement against input external energy, which can occur when thecircuit arrangement is being supplied in a relatively long distance.Even when the input external energy is at the maximum level to beassumed, the voltage which is built up across the light-emitting diodemust remain less than the forward voltage of the light-emitting diode.This prevents the light-emitting diode from starting to illuminateinadvertently when external energy is flowing—an interference voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail in the following textusing illustrations in the figures, in which:

FIG. 1 shows an outlet circuit diagram of a circuit arrangement foroperating an illuminated sign.

FIG. 2 shows a first embodiment of a drive circuit.

FIG. 3 shows a characteristic profile of the drive circuit shown in FIG.2.

FIG. 4 shows a second embodiment of a drive circuit.

FIG. 5 shows a characteristic profile for a drive circuit as shown inFIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a general configuration of an LED signal with n drivers T,with each driver T controlling at least one LED D. At least one LEDcontroller St is connected in parallel with each of the LEDs D. One poleof the LED controller St is connected directly to the operating voltageU, while the other pole is connected in series with a resistor R to theoperating voltage U. The resistor R is distinguished by having a definedfailure behavior, that is to say certain defects, for example completeshort circuits, are so improbable that, effectively, they do not occur.The resistors R are furthermore designed such that faults, for example ashort circuit, of the LED D or LED controller St have only a minoreffect on the overall current drawn by the circuit arrangement. Forexample, in the event of a short circuit of the LED controller St in thecase of an LED signal with 60 drivers T, the total current wouldincrease by only about 5%.

A first embodiment of a driver T is shown in FIG. 2 as a detail of theLED signal from FIG. 1. The LED controller St sets a current I_P inparallel with the LED D. This is done by using a controlled currentsource. A comparator, in this case an operational amplifier OPV, isconnected on the input side to a switching threshold Sch, whichrepresents a nominal value, and to an actual value sensor Act. On theoutput side, the operational amplifier OPV acts on a parallel path tothe LED D. The switching threshold Sch predetermine when the parallelcurrent I_P should be switched for daytime operation, or night operationor to an idle level. The switching threshold Sch itself has temperaturecompensation Tk and forward voltage compensation Fk applied to it. Thetemperature compensation Tk compensates for temperature-dependentcomponent characteristics, while the forward voltage compensation Fktakes account of LED-specific forward voltages. The actual valve sensorAct processes the operating voltage U and/or the operating current ofthe entire LED signal, although the actual value can also be preset byfurther information, such as additional control lines from a controlmechanism or else by information which is encrypted in the supplycurrent or in the supply voltage U. The current source for the parallelcurrent I P is designed such that it can produce a current even at lowvoltages. In consequence, input external energy, whose order ofmagnitude depends to a major extent on the length of the supply line, isshort-circuited in such a way that it is impossible for a high voltageto build up, or for the LEDs D to start to illuminate as a result of theexternal energy.

FIG. 3 shows the supply-voltage-dependent current profile of theparallel current I_P and of the current I P flowing through the LED D incomparison to a conventional incandescent lamp G. The figure shows aresistance line W, which is governed by the resistors R, as would occurif only the resistors R were present, and there were no drivers T orLEDs D. The resistance line W describes the maximum possible currentflow through the circuit arrangement according to FIG. 2 as a functionof the operating voltage U. With this circuit embodiment, the voltage isreduced at night by controlling the parallel current I_P. As can be seenfrom FIG. 3, the LED current I_D is not constant either in the daytimeor in the night time voltage range.

A circuit embodiment with an additional LED current controller issuitable for achieving a constant current flow. A circuit arrangementsuch as this is shown in FIG. 4. In this case, the parallel current I_Dand, in addition, the LED current I_D as well are controlled by means ofcontrolled current sources. FIG. 5 shows the associated current profile.The current that is set and hence the signal brightness are ideallyconstant during daytime operating as well as during night timeoperation.

When the signal is switched off, 0<=U_night min, the signal does notilluminate even when input external energy is present. The LED currentI_D is =0. However, the parallel current I_P ideally corresponds to themaximum possible current. Any input external energy is short-circuitedby this parallel current I_P, so that it is impossible for any voltageto build up that will cause the LEDs D to illuminate.

During night time operation U_night min<=U_night max, the LED controllerSt sets the LED current I_D=I_D_night through the LED D. The LEDs Dilluminate with a low light power level. The parallel current I_Pideally corresponds to the difference between the current which isdefined by the resistors R and LED current I_D.

During daytime operation U_day min<=U_day max, the LED controller Stsets the LED current I_D=I_D_day through the LED D. The LEDs Dilluminate with the maximum light power. The parallel current I_P onceagain corresponds to the difference between the current which wasdefined by the resistors R and the LED current I_D.

In the overlapping area between daytime and night time operation, thehysteresis for the parallel current I_P and the LED current I_D resultsin the LED signal remaining in a stable state.

The circuit arrangement shown in FIG. 4 represents a very convenientembodiment. The LED signal is safe in the signalling sense of the word,has high stability, is virtually free of temperature and componentfluctuations, and provides a very wide dynamic range between daytime andnight time operation. However, this convenience involves acorrespondingly high level of circuitry complexity. This high level ofcomplexity can preferably be countered by means of integratedtechnology. Simplified versions are also feasible, providing only someof the functionalities, with reduced complexity.

One embodiment takes account of the fact that the maximum possible lightpower shall be achieved during the daytime while, however, at night thelight power is limited to a defined level. For this purpose, the LEDcurrent I_D is kept constant only in the range between U_night min andU_night max, while the maximum possible LED current I_D of I_P=0 is usedin the range between U_day min and U_day max.

A further embodiment relates to the configuration for DC and AC voltage.For this purpose, LED controllers St can be equipped with appropriaterectified diodes. When operating using AC voltage, it is worthwhiledesigning the LED controller St such that root mean square values areused for the comparison with nominal values, rather than instantaneousvalues of the actual values.

It is also possible to design an LED controller St for both DC voltageand AC voltage, since an LED controller St for AC voltage also operatesin the DC voltage mode. However, in this case, it should be rememberedthat different switching thresholds Sch may possibly be required. Therequired voltage type can be selected by means of externally accessibleprogramming.

The invention is not restricted to the exemplary embodiments specifiedabove. In fact, a number of variants are feasible which make use of theinvention even with the features being configured in fundamentallydifferent ways.

1. A circuit arrangement for operating a luminous signal, comprising: atleast one light-emitting diode, with which a resistor is connected inseries and a controller is connected in parallel, wherein the controllerhas a first controlled current source for presetting a parallel currentwhich reduces the current flowing through the light-emitting diode, asecond controlled current source for presetting a current flowingthrough the light-emitting diode, and the first controlled currentsource has a comparator whose inputs are acted on by a switching pointand an actual value sensor.
 2. The circuit arrangement as claimed inclaim 1, wherein a switching threshold is acted on by temperaturecompensation and/or forward voltage compensation.
 3. The circuitarrangement as claimed in claim 1, wherein to preset the parallelcurrent, the first controlled current source has a device for driving acurrent such that, when a maximum expected external energy is input, avoltage which builds up across the light-emitting diode is less than aforward voltage of the light-emitting diode.